This invention relates to electric initiators.
Electric initiators, or electro-explosive devices as they may also be called, are energy transducers which convert an electrical energy input into an explosive energy output. These devices include primers, detonators and squibs, which are structurally quite similar and differ from each other primarily in the level of explosive energy output.
Various types of electrical initiators are known in the art. One type has a metallic or plastic shell, an explosive charge and a pair of lead wires connected by a bridge wire or other bridge element. Devices of this type having a plastic shell or case are shown in various references including U.S. Pat. Nos. 2,802,421 and 2,996,987. Numerous references, including U.S. Pat. Nos. 2,086,532, 2,118,533 and 2,658,451, show two-lead initiators having a metallic shell. U.S. Pat. Nos. 3,128,703 and 3,173,367 disclose initiators of this type in which the shell can be either plastic or metallic. The initiator in U.S. Pat. No. 3,128,703 includes a shell and a shell liner, both of which can be made of either metallic or polymeric material. The bridge wire or other bridge element is usually connected directly to the ends of the two leads, although U.S. Pat. Nos. 722,913 and 1,590,364 show devices in which the bridge elements and the lead wires are connected to conductive metal strips.
Initiators having a single lead are also known. This type of initiator is fired by current flowing from the lead across a spark gap or through a bridge element to a grounded metallic shell or case. Devices of this type are illustrated, for example, in U.S. Pat. Nos. 2,921,520 and 2,964,835. These patents show a detonator plug assembly comprising a conductive pin and a coaxial conductive sleeve surrounding but insulated from the pin. The sleeve is in contact with a metallic shell or case. The pin and sleeve serve as terminals for the initiator. The device may have either a spark gap or a bridge element between the pin and the sleeve.
A two-lead initiator having a plastic shell is the safest of the above types of devices from the standpoint of risk of accidental initiation. One-lead devices are subject to accidental firing by either a short circuit or static discharge from the lead to the grounded shell. Short circuits are usually caused by a wire from another circuit touching the lead wire. Two-lead initiators having a metal shell are also susceptible to accidental initiation by static electricity, either by discharge from a lead to the shell or from shunted leads to the shell. The danger of accidental initiation from either static discharge or short circuit is greatly reduced in a plastic shell initiator of the two-lead type. However, this type of initiator has disadvantages. One is that lower pressures must be used in consolidating the explosive charge in a plastic shell device, because the plastic shell is ordinarily weaker than a metal shell. Another disadvantage of two-lead initiators generally, whether they have a plastic or metallic shell, is that they are difficult to assemble. Attachment of a bridge wire to the ends of two leads requires precise orientation of the bridge wire with respect to the leads while the leads are held in a fixed position. Handling and holding the lead wires in fixed position is cumbersome because of their length. Also, the bridge wire, which is of fine diameter, is easily broken or detached from one of the two leads during further assembly of the initiator.